Thermal treatment of sheets of magnesic metal



Patented Mar. 15, 1949 THERMAL TREATMENT OF SHEETS F MAGNESIC METAL Robert L. Hopkins, Arnold, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application March 19, 1948, Serial No. 15,818

3 Claims.

. This invention relates to improvements in the thermal treatment of rolled sheets of magnesio metal, i. e., magnesium and magnesium base alloy. Its purpose and object is to minimize or prevent distortion of such sheets occurring during thermal treatment. The invention is directed to those thermal treatments in which the individual rolled sheets, or portions cut therefrom, are formed into a stack having two more or less parallel surfaces formed by the outer rolled surfaces of the top and bottom elements of the stack, the other stack surfaces being defined by the edges of the stacked sheet elements. When such stacks are subjected to thermal treatment, they are usually positioned on the outer surface of the bottom element of the stack, and therefore the invention is described herein with reference to a stack so positioned. However, the stack may rest, during thermal treatment, on a stack surface defined by edges of the stacked elements or may be suspended, by a sling or simila-r means, in the heating chamber. The principles of this invention are useful regardless of the position of the stack during treatment.

For purposes of illustration only reference will be made herein to the accompanying drawing in which- Fig. l is a representation of a stack of sheet elements;

Fig. 2 is a View, partially in section, of one manner in which the methods of this invention may be performed; and

Fig. 3 is a partially sectioned view of one heat iiow retarding element useful in the practice of the invention.

As used herein and in the appended claims, the term "magnesio metal includes magnesium and alloys containing a preponderant amount of such metal, usually 70 per cent by weight or more. There are several types of thermal treatment which may be applied to rolled sheet of magnesio metal. Usually such treatments involve heating the sheets to elevated temperatures, such as 200 to 850 F., and thereafter cooling said sheets. The purpose of the treatment is not involved in this invention, the invention being useful whenever such treatments are performed. When rolled sheets of magnesio metal are so heated and cooled, the sheet is often distorted to such a degree as to destroy the essential atness of the sheet. In such cases it is often found necessary to resort to further operations, such as stretching, to eliminate the distortion and restore the sheet to its original flatness.

It is the purpose of this invention to minimize or prevent such distortion and that end is accomplished by so controlling the iiow of heat during thermal treatment as to insure that the greatest rate of heat iiow into, or out of, the stack of sheet elements is in a direction substantially normal to the rolled surfaces of said sheet elements. In other words, the flow of heat into, and out of, those stack surfaces which are defined by the edges of the stacked sheet is so retarded that the heat ow is at its greatest rate through those surfaces of the stack which are dened by the rolled surfaces of the top and bottom sheet elements thereof.

`The principle of the invention will be apparentv from a consideration of the sheet stack shown in Fig. 1. Stack surfaces such as A are defined by the edges of the stacked sheet elements. Stack surfaces B are defined by the rolled surfaces of the top and bottom elements of the stack. In thermally treating such a stack in accordance with the principles of this invention the heat ilow is retarded at stack surfaces such as A so that the heat flows at its greatest rate in the direction indicated by the arrows. The condition illustrated in Fig. 1 is the cooling cycle of a thermal treatment. During the heating cycle of the treatment the heads of the ow arrows would point in the opposite direction. This control of heating and cooling results in a more uniform temperature in any given sheet than has been obtained heretofore.

The arrangement shown in Fig. 2 is typical of a preferred industrial application of the invention. The stack I0 of rolled magnesio metal sheets is supported upon a transporting device consisting of a frame built of structural elements Il on which is placed a platform i2. All or a portion of this platform may, if desired, be in the form of a grating, as indicated at I3. Also arranged on this transporting device are heat iiow retarding elements I4 which in this case are shown in the form of ingots or billets from which magnesio sheet is rolled. Since such ingots are always available in a rolling plant, they are conveniently used for this purpose. The assembly shown in Fig. 2 may readily be lifted and placed in a heating inclosure, such as a furnace.. When subjected to heat the passage of the heat into the surface A of the stack I0 will be retarded by the shielding eifeot of the ingots i4. Also when the assembly is removed from the furnace, or the furnace heat is shut off, to promote cooling of the stacked sheets, the heat stored in the ingots M will prevent rapid :flow of heat through the stack surfaces A. The result is therefore that the ingots it act as barriers and the heat flows most rapidly as indicated in Fig. l. In the preferred form of the invention, as shown in Fig. 2 hereof, a magnesium plate I8 is inserted between stack lil and the grating I3 to minimize any marking of the bottom elements of the stack by the grating and also to minimize any distortion effects due to the unequal expansion rates of the grating and the stack.

It will be at once apparent that many various means may be used in the practice of this invention to produce the desired heat ilo-w conditions. For instance, the heat retarding barrier may be formed of material either of substantial volume, such as the ingots Hl, or of relatively high insulating properties. A form of device useful for this purpose is the refractory block i5 shown in Fig. 3. Imbedeied in block I5 are heating elements i6 to which electrical energy may be supplied through leads il. Several cf such blocks may be used to enclose the stack surfaces A during heating and then, during cooling, energy supplied to the heating elements i6 will insure that flow cf heat in a direction normal to stack surfaces A will be retarded. Any other barrier or heat flow retarding device may be used, in effecting, in accordance with this invention, a retarding of the flow of heat into, and out of, those surfaces of the stack which are dened by the edges of the stacked magnesic metal sheets.

The principles of this invention have been successfully used, in the embodiment shown in Fig. 2, to substantially minimize, and often prevent, distortion of rolled magnesio sheet during thermal treatments applied to thestacked sheet and have greatly reduced the need and frequency of later operations designed to restore flatness t0 the sheet.

Having thus described my invention, I claim:

1. The method of minimizing distortion of rolled magnesio metal sheet during thermal treatment applied to a stack of such sheets which comprises retarding the flow of heat through those surfaces of said stack which are dened by the edges of said sheets thereby causing a greater rate of heat flow in a direction normal to the other surfaces of said stack.

2. In a method of thermally treating rolled magnesio metalsheet in which a plurality of said sheets are formed into a stack and submitted to a heating and a cooling cycle, the improvement consisting in placing heat flow retarding elements adjacent those surfaces of said stack which are defined by the edges of the stacked sheet thereby to retard the ow of heat intoa'nd from said stack surfaces to a rate less than: the rate of heat ow in a direction substantially normal to the stack surfaces dened by the outer' rolled surfaces of the top and the bottom. ele'-a ments of said stack.

elements of Y a through the edges of said stacked elements dure ing said treatment.

ROBERT L. HOPKINS.

No references cited. 

